The field of the invention is that of telecommunications networks.
It finds non-limiting preferred applications in Voice over Internet Protocol (VoIP) telecommunications networks.
In Voice over IP networks in particular, users register periodically to inform the network of their presence, thus providing an address at which they can be contacted. In such networks in particular, it is usual for user terminals to access the network via a home gateway. As known in itself, this gateway includes a table in which it associates the IP address and the port associated with each of the terminals of the home network with an address and a port of that same gateway on the public network.
This table is known to persons skilled in the art as a network address translation (NAT) table. Here the term NAT is to be understood in a broad sense as encompassing the wider concept of network address and port translation (NAPT).
As is known in the art, gateways regularly delete the content of the address translation tables if they are not updated. One known method of keeping the address translation tables up to date is to force artificial exchanges of signaling between the user terminal and the network, as described below with reference to FIG. 1.
This figure shows four exchanges of messages L1 to L4 by this mechanism. In this figure:                the reference 10 designates a terminal;        the reference 20 designates a home gateway or a router modem including a NAT table;        the reference 30 designates a user collection unit able to communicate with the home gateway; and        the reference 40 designates a network core unit able to manage user registration for a given service, for example Voice over IP, such as a Session Initiation Protocol (SIP) server.        
With reference to the exchange of messages L1, the terminal 10 sends the network core entity 40 a request REG (3600) to register for one hour.
These messages pass through the home gateway 20 and the user collection unit 30, both of which are in series between the terminal 10 and the SIP server 40.
On reception of this message, the SIP server 40 authorizes maintaining the registration of the user in its databases for one hour by sending a message OK (3600).
In the prior art, the value of the subscription provided by the SIP server is then one hour.
To force the exchange of signaling linked to the registration of the user of the terminal 10 in order for the information to continue to be stored in the address translation table, the user collection unit 30 intentionally reduces the value of the subscription provided by the network by sending the terminal 10 via the home gateway 20 an authorization message of subscription value that is set at five minutes (300 seconds) in this example.
Consequently, and as shown for the exchange of messages L2, substantially 300 seconds after receiving the message modified by the user collection unit 30, the terminal 10 requests from the SIP server 40 registration for 300 seconds. It is known in the art that the Voice over IP terminals 10 adapt to the authorizations provided by the network core.
Of course, it is not necessary for the user collection unit 30 to route this registration request to the network core 40 because, in the example described here, the said core has already agreed to maintain the subscription of the terminal 10 for a subscription period of one hour.
Consequently, the user collection unit 30 merely sends the terminal 10 a new subscription authorization for a further 300 seconds.
The mechanism described for the exchange of messages L2 is repeated identically for the exchange of messages L3, as shown by the double-headed dashed-line arrow.
As is known in the art, the user collection unit 30 monitors in parallel with the above process whether the subscription provided by the SIP server 40 (which is one hour in this example) is approaching its expiry.
Assume that this is so when the user collection unit 30 receives the request for registration for five minutes during the exchange of messages L4.
The user collection unit 30 then replaces the value of the subscription minus 300 seconds in this registration message with the original subscription value of 3600 seconds, in order to solicit a new subscription of the terminal 10 with the SIP server 40 of one hour.
As described above with reference to the exchange of messages L1, the SIP server 40 agrees to a subscription of one hour and the user collection unit 30 substitutes in this message for the value of the subscription provided by the network core a reduced value set at 300 seconds, in order artificially to force exchange of signaling by the terminal 10 to maintain the address translation table in the home gateway 20 up to date.
A first drawback of the above mechanism is that it artificially generates superfluous application signaling message traffic in the access network, which obliges the operator to double the capacity of the Voice over IP service user collection units.
A second drawback of this mechanism is that it requires the introduction into the collection units of functions specific to generating this artificial traffic.